Magnetic head-based systems have been widely accepted in the computer industry as a cost-effective form of data storage. In a magnetic disk drive system, a magnetic recording medium in the form of a disk rotates at high speed while a magnetic read/write transducer, referred to as a magnetic head, “flies” slightly above the surface of the rotating disk. The magnetic disk is rotated by means of a spindle drive motor. The magnetic head is attached to or formed integrally with a “slider” which is suspended over the disk on a spring-loaded support arm known as the actuator arm. As the magnetic disk rotates at operating speed, the moving air generated by the rotating disk in conjunction with the physical design of the slider lifts the magnetic head, allowing it to glide or “fly” slightly above and over the disk surface on a cushion of air, referred to as an air bearing. The flying height of the magnetic head over the disk surface is typically only a few tens of nanometers or less and is primarily a function of disk rotation, the aerodynamic properties of the slider assembly and the force exerted by the spring-loaded actuator arm.
In a magnetic tape drive system, a magnetic tape typically containing data tracks that extend along the length of the tape is drawn across magnetic tape heads. The magnetic tape heads can record and read data as relative movement occurs between the heads and the tape.
A major problem that is encountered during manufacturing, handling and use of magnetic recording transducers, referred to as heads, is the buildup of electrostatic charges on the various elements of a head or other objects which come into contact with the heads, particularly heads of the thin film type, and the accompanying spurious discharge of the static electricity thus generated. Static charges may be externally produced and accumulate on instruments used by persons performing any head manufacturing. These static charges may be discharged through the head causing excessive heating of the sensitive sensors which result in physical damage to the sensors.
Magnetoresistive sensors, also referred to as “MR heads,” are particularly useful as read elements in magnetic heads, especially at high data recording densities. Two examples of MR materials used in the storage industry are anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR). The MR sensor provides a higher output signal than an inductive read head. This higher output signal results in a higher signal-to-noise ratio for the recording channel and allows higher areal density of recorded data on a magnetic surface of the media.
As described above, when a head is exposed to electrostatic discharge (ESD), or even a voltage or current input larger than that intended under normal operating conditions, referred to as electrical overstress or EOS, the sensor and other parts of the head may be damaged. This sensitivity to electrical damage is particularly severe for MR read sensors because of their relatively small physical size. For example, an MR sensor used for extremely high recording densities are patterned as resistive sheets of MR and accompanying materials, and will have a thickness for one of the sheets on the order of 100 Angstroms (Å) by 1 to 10 microns (μm) and a height on the order of 1 μm. Discharge currents of tens of milliamps through such a small resistor can cause severe damage or completely destruction of the MR sensor. The nature of the damage which may be experienced by an MR sensor varies significantly, including complete destruction of the sensor via melting and evaporation, oxidation of materials at the air bearing surface (ABS), generation of shorts via electrical breakdown, and milder forms of magnetic or physical damage in which the head performance may be degraded.
While a disk head is comprised of a single MR element, modem tape heads have multiple MR elements, on the order of 8 to 32, or even more, all of which must be good. The large number of MR sensors in a tape drive, and thus, the significantly higher cost, makes testing during manufacturing more important and ESD loss due to a single element is very expensive as the entire head must then be scrapped.
Prior solutions to ESD and EOS protection can be summarized into two types of approaches: 1) by using diode(s) and 2) by shorting out the sensor element. However, both of these approaches have significant disadvantages. In the diode approach, the diode is intended to remain in parallel with the sensor element during normal operation of the disk (or tape) drive. The current flowing through the diode during normal operation must be small in order for the diode to not affect the operating effectiveness of the sensor element. Common bias voltages for MR heads are in the range of 350 mV to 700 mV, which is the regime over which a diode begins to conduct current. This leaking current through the diode leads to noise, which will lower the signal to noise ratio of the readback process. Diodes also introduce parasitic capacitance across the head, and from the head leads to the electrical ground, which adversely affects the maximum readback bandwidth achievable with the head.
Electrically shorting out the MR sensors, by shorting the two ends of the sensor which connect to external devices, provides the best possible ESD protection. The problem with this technique is that it is no longer possible to test the head after the short is applied. Ideally, the short is applied early in the head fabrication process, and not removed until the disk or tape drive is assembled. Due to the cost of a head as it progresses through wafer fabrication, slider/module fabrication, suspension mounting, and head assembly build, it is beneficial to be able to test the head at various points to determine whether to continue using that head.
Elser et al. U.S. Pat. No. 4,317,149 discloses an inductive head having short discharge paths formed by the deposition of conductive material in recesses formed in an insulating layer so that the static electric discharge will occur in areas displaced from the critical pole tip and gap area at the slider air bearing surface.
Schwartz et al. U.S. Pat. No. 4,800,454 discloses an inductive head assembly wherein the magnetic pole piece and the inductive coil winding are coupled to the slider to allow discharge of any static electric charges which may build up. The winding is connected to the slider body via a diode with high forward and reverse voltage drops, or through a fusible link.
U.S. Pat. No. 5,465,186 describes an approach for protecting a magnetic read/write transducer from the effects of electrical overstress and electrostatic discharge by shorting out the conductive leads of a magnetoresistive (MR) sensor element to provide a low resistance, conductive path that bypasses the MR element and minimizes electrical current through the MR sensing element during discharge of static electrical charge. The MR sensor lead terminal pads are shorted together by soldering or by using twisted conductor pairs. The other transducer elements such as the MR magnetic shields, the inductive coil and the inductive magnetic yoke structure may also be shorted to the MR sensor leads by soldering the lead terminal pads together at the slider surface. Remotely located protective devices, such as reversed diode pairs, can also be connected across the MR sensor element using the twisted pair. However, since it is sometimes necessary to measure the resistance or other parameters of the device during fabrication, the shorting device must be removed, exposing the device to possible ESD/EOS damage.
U.S. Pat. No. 5,491,605 describes a scheme for protecting a magnetic read/write transducer from EOS and ESD. The elements of the MR and inductive heads are shorted together and to the slider substrate by depositing a conductive material layer, such as tungsten, over the slider air bearing surface to provide a low resistance, conductive path that minimizes current through the MR element during discharge of electrostatic charge. The conductive layer is removed by wet etching prior to placing the magnetic head into operation in a magnetic storage system.
A switchable short was described in U.S. Pat. No. 5,465,186 that would allow the short to be temporarily opened for testing. However, this method is difficult to realize, as switches require large amounts of real estate on the back of the slider, and the switching process requires low resistance shorting and re-shorting structures. Switches can also be expected to last for only a limited number of opening and closing cycles.
Another method of protecting the head is to add crossed diodes to the cable connecting the leads of each read head and thereby clamping the maximum voltage across the leads. Since modem tape heads have multiple read elements, it can be expensive to add diodes to each cable, particularly where the head and cable are scrapped during the testing phase.
While mounting disposable diodes on a single slider may be cost effective, the shear number of diodes required for a modem tape head adds significant cost to the head if the diodes are not reusable by multiple heads during manufacturing. Furthermore, the added weight of many diodes or chips on the cable will affect the dynamics of the head actuation, potentially degrading its track following performance.
A need therefore exists for providing reusable ESD and EOS shunt protection for a read/write head assembly that would allow the MR read/write head to be tested at various manufacturing stages without the aforementioned disadvantages.